This invention relates to a unique method and means for producing highly charged droplets at a high rate and with low power requirements and using these droplets to remove components of a gaseous effluent. The highly charged droplets are caused to drift, by means of an electric field, through the gaseous effluent, absorbing selected gases and aerosol particles therefrom, and carrying them to a collecting electrode.
Typical of the components removable from gaseous effluents are SO.sub.x and NO.sub.x.
Most of the present SO.sub.x /NO.sub.x scrubbing systems, especially those capable of treatment of volume flow rates greater than about 2000 cfm can be classified into the following basic categories: venturi scrubbers, floating bed, packed bed, spray tower and tray column. All of these systems are characterized by one or more disadvantage, such as a high liquid-to-gas ratio, high pressure drop, scale build-up, low scrubbing efficiencies, or in requiring mist eliminators.
Thus, venturi scrubbers require the effluent to be forced through a venturi throat at velocities of 200 to 400 ft/sec. Part of the energy in the high velocity effluent flow is used to produce small droplets by tearing them off of spray nozzles which project into the flow. Initially, these gas have a high relative velocity with respect to the droplets, but the viscous drag forces rapidly accelerate the droplets to the speed of the effluent. The droplets must be subsequently removed by the use of a cyclonic precipitator, a wet electrostatic precipitator, a mist eliminator or a combination thereof. In passing through a venturi scrubbing system, the effluent undergoes a pressure drop of from 15 to 20 in. water.
For a power plant producing 1,000 MW the power required just for the compression of the effluent for such a plant would be about 5 MW or 0.5% of gross capacity. It has been determined that such systems actually require about 2.5% of gross capacity, or about 8 KW/1000 cfm.
Floating bed scrubbers suffer from scale build-up, high liquid-to-gas ratios (.about.50 gal/1000 cfm) and significant pressure drops (.about.15 in. H.sub.2 O). Packed bed scrubbers have similar disadvantages.
Spray towers require about 80 gal/1000 cfm representing a power loss of about 1% of the gross capacity of a power plant. Additionally, the pressure drop in a spray tower is about 10 in. water, representing another 1% real power loss of the gross capacity of the plant. Tray columns have similar disadvantages.
Most of the aforesaid scrubbing systems require the use of a mist eliminator downstream from the scrubber to remove the smaller droplets of liquor which are entrained in the gas flow, which eliminators are quite susceptible to scaling and plugging problems.
Such "mist" droplets are produced when the liquid jet from the spray nozzles breaks up. When a droplet begins to break off from the end of the jet and becomes an entity, the neck also breaks up into a much smaller, mist droplet. It is these droplets which are easily entrained in the gas flow because of their small size, and are very difficult to remove.
With my invention, all of the droplets are highly charged, thus ensuring that they will be removed in the collecting chamber. Furthermore, the energy required to produce my highly charged droplets is very small; e.g., less than one watt is required to produce the droplets necessary for scrubbing 1000 cfm.
The large pressure drops required by conventional scrubbers is not required with my invention since the flow through the chamber is unobstructed and does not require high velocities.
A further, and important, characteristic of my invention is that the droplet sizes and drifting velocities can be controlled to ensure that saturation is achieved in all of the droplets.
Summarizing the shortcomings of previous methods for using charged particles for removing oppositely charged particles and certain gases, it has been found that one or more of the following undesirable characteristics were present:
(1) the spray nozzles used to produce the charged droplets also produced corona discharges, either from the nozzle itself or the stream of water extending from it, thus consuming power and flooding the gas with ions; (2) a significant percentage of the droplets produced were either uncharged or had such a small charge that they could not be effectively drifted by an electrical field; (3) the drop size distribution either could not be controlled or was too broad (containing very small droplets, less than 10 micron diameter, as well as large droplets, greater than 1 millimeter in diameter); (4) nozzles are used in conjunction with a venturi throat, thereby requiring large power consumption, and (5) the velocity and trajectory of the droplets, even those charged, could not be controlled, thus allowing many droplets to escape from the device.